The present disclosure relates to a system and a method for cooling an optical fiber, such as a fiber laser and/or amplifier.
Gain media based on optical fibers (e.g., fiber lasers and amplifiers) provide a broad range of performance features, including high efficiency, robust single-mode output, high reliability, compact coiled packaging, large surface-area-to-volume ratio for favorable thermal performance, and an all-fiber architecture without any free-space optics and hence no requirement for a rigid optical bench.
Over the past decade, output powers of fiber lasers have been increased several orders of magnitude, from watt-level to 10 kW. When a system incorporating these high-power fiber lasers is built, then a means for cooling such high-power fiber lasers is needed.
Thermal management of solid-state lasers is quite mature, and the basic cooling approaches are known. However, many conventional cooling approaches include moving parts, flowing liquids, etc., and are therefore not desirable for use in space-based applications.
Further, conventional cooling approaches that are used for relatively short, wide solid-state lasers may be very different from cooling approaches that are suitable for long, thin fiber medium (e.g., fiber lasers and amplifiers). For example, in fiber laser cooling, the long length of the fiber makes it difficult to maintain intimate thermal contact between a heat sink and the fiber over wide temperature ranges, mainly due to differences in thermal expansion of the glass fiber and the metal heat sink. Therefore, it is desirable to have a cooling approach for fiber lasers in which good thermal contact is maintained between the fiber laser and a heat sink which conducts heat away from the laser medium to a heat exchanger, or a radiator (e.g., in space-based applications). Also, it is desirable to have a cooling approach for the fiber lasers (e.g., used in space-based applications) in which the heat from the fiber laser is efficiently dissipated to the heat exchanger, or radiator (e.g., in space-based applications) without the use of moving parts, flowing liquids, etc.
Thermal management of high-power fiber lasers is not very mature, because kW-class fiber lasers (which require the most aggressive cooling) have only been available for the past few years. Commercial fiber lasers producing powers below about 50 W are typically formed into a coil and then cooled by ambient air. For higher powers of several hundred watts, and perhaps up to the kW level, the coils are water-cooled. For example, kW-class fiber lasers may be cooled simply by immersing the coiled fiber in a water bath.
One conventional approach uses phase change materials (PCMs) for cooling fiber lasers in which small capsules of PCM are suspended in a liquid coolant, which is in direct thermal communication with the fiber. The small capsules of PCM are suspended in the liquid coolant to increase the heat capacity of the liquid. Then the liquid coolant is allowed to flow longitudinally along the length of the fiber. However, the PCM capsules may not flow at the same rate as the liquid, and in fact the PCM capsules may actually be static; in these cases the PCM capsules do not provide the anticipated thermal management benefit. Heat is removed from the PCM capsules by the coolant liquid. This cooling approach uses flowing liquids and moving parts that are not optimal for space-based applications.
Another conventional approach for cooling of fiber lasers uses a cooling approach in which the fibers are wrapped about a cylindrical heat sink. This cooling approach does not use phase change materials. One design strategy is to avoid placing the fiber under tension, due to concerns that sustained tension will affect the optical properties of the fiber or promote early failure of the fiber. For this reason, winding the fiber on the outside of the metal cooling ring is avoided, since the winding will have to be done with a certain amount of tension in the fiber to overcome the natural tendency of a fiber coil to spring out away from the coil axis. This tendency would likely reduce the fiber thermal contact with the heat sink. To overcome this problem, the fiber may be wound around the inside of a cooling ring. A spiral groove may be cut into the internal surface of the cooling ring, and the fiber is wound into the groove and thermally contacted to the ring by means of grease, liquid, or a gel. This accommodates a difference in thermal expansion between the fiber material and the cooling ring by making the inner grooves deep enough and filling them with a compliant thermal-contact medium, such that any significant coefficient of thermal expansion (CTE) mismatch can be accommodated by allowing the fiber to move radially in the grooves. This conventional cooling approach is not optimal for space applications, due to the requirement for grease, liquid, or gel to provide thermal contact.
The present disclosure provides improvements over the prior art methods and systems for cooling optical fibers.